This invention relates generally to fluidized bed boilers.
One type of boiler employs fluidized particles as a heat exchange medium. In such boilers, inert particles, such as sand, are placed in a fluidized state by air delivered from below through a porous plate or a tubular grid structure. The air also provides at least a portion of the oxygen required to burn fuel within the bed for heating the particles. In order to initiate the combustion process, an auxiliary burner may be provided to preheat the particulate material.
The particulate material represents a large mass which must be heated at start-up to a level sufficient to safely ignite and sustain combustion of the primary fuel. However, the high heat transfer coefficient existing between the bed and the transfer surface in fluidized bed systems has made start-up and low load operations difficult and somewhat dangerous, especially in steam or hot water boiler applications. Because conventional fluidized beds tend to rapidly reject heat, a significant heat input is required to reach the desired bed temperature. In some cases, heat inputs of up to twenty percent of the boiler rating are necessary to achieve primary fuel combustion temperature. Even when the temperature of the fluidized bed reaches or exceeds this temperature, there may still be great difficulty in maintaining bed temperature if boiler load is rapidly applied and if the start-up heat source is discontinued. It is not unusual for such a system to require heat inputs of up to thirty to forty percent of maximum load before the start-up burner may be safely shutdown.
Prior art attempts at achieving start-up and low load operation of fluidized bed systems have included the use of multiple cells or chambers, each containing a portion of the total system mass of bed material. This permits the bed material to be heated in stages during the start-up process. Another prior art system includes apparatus for the withdrawal of bed material to allow start-up and low load operation with a shallow bed and the delivery of additional bed material into the system as the temperature and load are increased. Such systems are not wholly satisfactory because they are relatively expensive and involve complex control considerations. Also, because of the necessity to remove and store or dispose of relatively hot bed material when the load is decreased, the material transport system presents safety hazards.